fix: world_offset precision fix for large-coordinate DXF files (e.g. Turkish UTM)

Subtracts the DXF header extents centroid (world_offset) from all coordinates
in f64 BEFORE casting to f32. This eliminates the ~0.5 m ULP error that occurs
at 4,000,000 m UTM coordinates and caused geometry to disappear on zoom-out.

- Scene: adds world_offset field + compute_and_set_world_offset() method
- tessellate.rs: threads world_offset through all f64→f32 conversions
  (legacy_geometry, dimension_geometry, solid_wire_fallback, multileader)
- truck_tess.rs: to_local() helper already in place from prior session
- mod.rs: tessellate_entity() and entity_aabb() apply world_offset
- update.rs: compute_and_set_world_offset() called after doc load
- cmd_result.rs / solid3d_cmds.rs: pass correct offset to truck tessellators

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/update.rs

@@ -73,6 +73,7 @@ impl H7CAD {
                     }
                 }
 
+                self.tabs[i].scene.compute_and_set_world_offset();
                 self.tabs[i].scene.populate_hatches_from_document();
                 self.tabs[i].scene.populate_images_from_document();
                 self.tabs[i].scene.populate_meshes_from_document();